V. K. Agrawal

SWIFT view of the 2015 outburst of GS 2023+338 (V404 Cyg): complex evolution of spectral and temporal characteristics

We study the spectral and temporal characteristics of the source GS 2023

DETECTION OF GRB 090618 WITH THE RT-2 EXPERIMENT ON BOARD THE CORONAS-PHOTON SATELLITE

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ENERGY-DEPENDENT TIME LAGS IN THE SEYFERT 1 GALAXY NGC 4593

338 (V404 Cyg) during the initial phase of its June 2015 outburst, over the energy range of 0.5 - 150 keV. This is the first detailed study of the characteristics of this source based on {\it SWIFT} observations, being reported. Based on our analysis, we understand that the source existed in the hard, intermediate and soft spectral states. We find that the evolution of the spectral parameters, the hardness intensity diagram and the rms-intensity diagram are not similar to those observed for most of the outbursting black hole sources. We also observe presence of weak peaked components in the power density spectra during the intermediate state of the source. Dramatic changes in the spectral and temporal properties are also exhibited before the ejection of a radio jet suggesting it to be associated with the coronal mass ejection. It seems that may be due to evacuation of the inner part of the Keplerian disc for a short duration, the disc component is not observed after the huge radio flare. The absorption features observed in the low energy spectra suggest the presence of wind emission and the evolution of the characteristics of the variable Fe line emission during both hard and intermediate states, indicate its origin to be probably related to the wind/outflow.

RT-2 DETECTION OF QUASI-PERIODIC PULSATIONS IN THE 2009 JULY 5 SOLAR HARD X-RAY FLARE

We present the results of an analysis of the prompt gamma-ray emission from GRB 090618 using the RT-2 Experiment on board the Coronas-Photon satellite. GRB 090618 shows multiple peaks, and a detailed study of the temporal structure as a function of energy is carried out. As the gamma-ray burst (GRB) was incident at an angle of 77? to the detector axis, we have generated appropriate response functions of the detectors to derive the spectrum of this GRB. We have augmented these results using the publicly available data from the Swift Burst Alert Telescope detector and show that a combined spectral analysis can measure the spectral parameters quite accurately. We also attempt a spectral and timing analysis of individual peaks and find evidence for a systematic change in the pulse emission characteristics for the successive pulses. In particular, we find that the peak energy of the spectrum, Ep , is found to monotonically decrease with time, for the successive pulses of this GRB.

Discovery of a quasi-periodic oscillation in the ultraluminous X-ray source IC 342 X-1: XMM–Newton results

We investigate the energy-time lag dependence of the source NGC 4593 using XMM-Newton/EPIC pn data. We found that the time lag dependency is linear in nature with respect to the logarithm of different energy bands. We also investigate the frequency-dependent time lags and identify that at some frequency range (5 × 10–5 Hz to 2 × 10–4 Hz) the X-ray emission is highly coherent, mildly frequency dependent, and very strongly energy dependent. These observations can be explained in the framework of the thermal Comptonization process, and they indicate a truncated accretion disk very close to the black hole. We discuss the plausible spectral state to explain the phenomenon and conclude that the observed properties bear a close resemblance to the intermediate state or the steep power-law state, found in galactic black hole sources.

‘Spectro-temporal’ variabilities and possible physical mechanism for jet ejections

We present the results of an analysis of hard X-ray observations of the C2.7 solar flare detected by the RT-2 experiment on board the Coronas-Photon satellite. We detect hard X-ray pulsations at periods of {approx}12 s and {approx}15 s. We find a marginal evidence for a decrease in period with time. We have augmented these results using the publicly available data from the RHESSI satellite. We present a spectral analysis and measure the spectral parameters.

Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

We report the discovery of a quasi-periodic oscillation (QPO) at 642 mHz in an XMM–Newton observation of the ultraluminous X-ray source (ULX) IC 342 X-1. The QPO has a centroid at νQPO = 642 ± 20 mHz, a coherence factor Q = 11.6 and an amplitude (rms) of 4.1 per cent with significance of 3.6σ. The energy dependence study shows that the QPO is stronger in the energy range 0.3–5.0 keV. A subsequent observation (6 d later) does not show any signature of the QPO in the power-density spectrum. The broad-band energy spectra (0.3–40.0 keV) obtained by quasi-simultaneous observations of XMM–Newton and NuSTAR can be described well by an absorbed diskbb plus cutoffpl model. The best-fitting spectral parameters are power-law index (Γ) ∼ 1.1, cut-off energy (Ec) ∼ 7.9 keV and disc temperature (kT_in) ∼ 0.33 keV where the QPO is detected. The unabsorbed bolometric luminosity is ∼5.34 × 1039 erg s−1. Comparing this with the well-known X-ray binary GRS 1915+105, our results are consistent with the mass of the compact object in IC 342 X-1 being in the range ∼20–65 M⊙. We discuss the possible implications of our results.